1. Field of the Invention
The present invention relates to an illumination device, and more particularly, to an illumination device disposed near a side of a liquid crystal panel.
2. Related Art
Illumination devices also known as front lights are positioned on a front side of a reflection type liquid crystal display. The front lights are positioned above a viewer""s side of a liquid crystal panel to illuminate a liquid crystal panel.
FIG. 12 is a sectional view of a liquid crystal display 100 having a front light 110 positioned on a front side of a liquid crystal panel 120. In the illustrated liquid crystal panel 120, a liquid crystal layer 123 secured by a sealant 124 is positioned between a top substrate 121 and a bottom substrate 122. A liquid crystal control layer 126 is positioned on an inner surface of the top substrate 121. A reflection layer 127 having a high reflectance is positioned below a liquid crystal control layer 128.
As shown, the front light 110 includes a flat light guide plate 112 and a light source 113 positioned directly adjacent to a side end face 112a. A portion of the light emitted from the light source 113 is received by the light guide plate 112 at the side end face 112a. The light is reflected by a reflecting surface 112c that includes a prism that changes the propagating direction of the light.
An anti-reflection layer 117 is positioned directly adjacent to an exit surface 112b, which allows light to be directed toward the liquid crystal layer 123. The anti-reflection layer 117 prevents reflected light within the reflection type liquid crystal panel 120 from being further reflected within the light guide plate 112.
In some devices a plurality of layers having different refractive indices such as layers made of SiO2 and TiO2 form the anti-reflection layer 117. This anti-reflection type layer is formed by a sputtering and vacuum deposition method. The method can provide a 1/4xcex optical condition, which allows light to be transmitted with a high transmittance ratio.
The above-described method for forming the anti-reflection layer 117 can have many problems. One problem is that the vacuum deposition and sputtering methods have a low yield and a high manufacturing cost. The high cost arises, in part, because these methods are processed in batch. Since the anti-reflection effect is provided by a combination of reflective indices and layer thicknesses, it can be difficult to achieve an anti-reflection effect for all visible wavelengths. Moreover, when an illumination device having such an anti-reflection layer is observed from a diagonal position, the anti-reflection layer 117 can appear with a colored tint that diminishes the quality of a displayed image.
Durability can also become problem since the above-described anti-reflection layer 117 is made of multiple layers. Multiple layers are especially susceptible to environments having a high temperature and a high humidity. Such conditions can affect the reliability of the light guide plate 112 and the front light 110.
To improve productivity, a method of making an anti-reflection layer has been proposed that uses an organic compound having a relatively low refractive index. In this concept, an immersion process uses a material whose refractive index can be arbitrarily changed and from which a practical processing liquid can be produced. Unfortunately, it is difficult to form an anti-reflection layer that can provide a high anti-reflection effect because there are few materials that can adequately control the refractive index and are easy to produce. Further, to achieve a practical anti-reflection effect, the application of the organic compound to the light guide plate must be followed by post-processes such as a heating process, which deteriorates the characteristics of the light guide plate.
A light guide plate comprises a structure that receives light at a side end face, facilitates light propagating therein, and conveys light through an exit surface. Preferably, an anti-reflection layer is coupled to the exit surface. In one embodiment, the anti-reflection layer comprises microscopic recesses and/or projections. Theses recesses and/or projections can be a submicron in lenght and/or arranged like a lattice on the exit surface.
In a light guide plate embodiment, microscopic concave and/or convex features about equal to or smaller than the wavelength of visible light are arranged or formed on the exit surface of a light guide plate. Preferably, the concave and/or convex features prevent light incident to an exit surface from being reflected, thereby improving the transmittance ratio at the exit surface. The light guide plate allows light propagating in the light guide plate to pass through the exit surface at a high efficiency. When combined with a light source, this embodiment encompasses a high intensity illumination display.
In a second light guide plate embodiment, the anti-reflection layer comprises microscopic recesses and/or projections preferably having a pitch of about 0.3 xcexcm or less. Preferably, this configuration allows shorter wavelength light to be sufficiently transmitted, thereby providing an anti-reflection effect. When the pitch exceeds 0.3 xcexcm in this embodiment, a portion of the light traveling though the light guide plate is reflected, which reduces the transmittance ratio at the exit surface. While the effect of preventing reflection of light in this embodiment becomes more significant the smaller the pitch, the pitch is preferably about 0.2 xcexcm.
In a third light guide plate embodiment, the microscopic recesses and/or projections are arranged in a staggered lattice. Such a configuration allows a higher density of recesses and/or projections than that of the tetragonal lattice arrangement and embodiment. Preferably, the microscopic recesses can be arranged with a small effective pitch, which improves the anti-reflection effect and prevents light transmitted or reflected by the anti-reflection layer from being tinted.
In a fourth embodiment, the recesses and/or projections formed in a staggered arrangement are preferably arranged in a direction in which the effective pitch of the recesses or projections is minimized in a main light guide direction. Such a configuration provides an improved anti-reflection effect.
xe2x80x9cAn effective pitchxe2x80x9d is equivalent to the distance between a first straight line that passes through the center of a certain projection (recess) and a second straight line that passes through the center of a projection (recess) adjacent to the projection (recess) and is parallel to the first straight line. The effective pitch of the plurality of projections arranged like a tetragonal lattice in an arranging direction of the projections is the same as the pitch of those projections. The effective pitch in the diagonal direction of the tetragonal lattice is xc2xd of the pitch of the projections and is slightly smaller than the actual pitch. Further, when the projections of the anti-reflection layer are arranged in a highest density in a staggered arrangement like a hexagonal lattice, the effective pitch is as small as about xc2xd of the actual pitch.
The xe2x80x9cmain light guide direction in the plane of the light guide platexe2x80x9d is a macroscopic propagating direction of light introduced into the light guide plate from the light source positioned near a side end face of the light guide plate. The main light guide direction is normally a direction from the side end face where the light source is positioned toward a second side end face that is across from it.
In one light guide plate embodiment, the effective pitch of the recesses and/or projections in the main light guide direction in the plane of the light guide plate is preferably about 0.15 xcexcm or less. Such a configuration provides a light guide plate that achieves an improved anti-reflection effect and which prevents light transmitted or reflected thereby from being tinted.
A method of manufacturing a light guide plate includes coupling a light source to a side end face having an exit surface. Preferably, the method utilizes an injection molding die that has a submicron lattice microscopic recesses and/or projections on a cavity wall associated with a light guide plate exit surface. Preferably, the shape of the microscopic recesses and/or projections within the light guide plate are formed by the injection molding.
Preferably, the manufacturing method forms the anti-reflection layer on the exit surface and the light guide plate together. In this aspect, there is no need for performing the separate acts of forming or applying an anti-reflection layer. The elimination of these acts improves the efficiency of the manufacturing process.
In one method of making a light guide plate, a die is used having microscopic recesses and/or projections in the form of a staggered lattice arranged and formed with a pitch of about 0.3 xcexcm or less. Preferably, the lattice is formed within a wall cavity of the die that forms the light guide plate exit surface.
An illumination device embodiment includes any of the above-described light guide plates and a light source positioned near a side face of the light guide plate. Preferably, light introduced into the light guide plate from the light source exits from the exit surface of the light guide plate.
Preferably, the illumination device embodiment prevents attenuation of light reflected by the liquid crystal panel caused by a reflection at a bottom surface of the liquid guide plate, which increases the panels intensity. When coupled to a color display, true color reproducibility is achieved because light transmitted by the light guide plate is not distorted by a tint.